The present invention relates to printing, and more particularly to the process of fusing toner to paper.
Modern laser printing is generally accomplished by what is commonly known as an imaging process. At the heart of the imaging process is an organic photoconductive (OPC) drum. The drum typically includes an extruded aluminum cylinder coated with a non-toxic organic photoconductive material. There are six generalized stages to the electrophotographic process: cleaning, conditioning, writing, developing, transferring and fixing.
Cleaning is the first stage in the electrophotographic process. This stage prepares the OPC drum to receive a new latent image by applying a physical and electrical cleaning process. The physical cleaning of the OPC drum is typically accomplished by a drum cleaning blade (or wiper blade) and a recovery blade. The wiper blade scrapes any excess toner from the drum and the recovery blade catches the toner and sweeps it into a waste hopper. In the electrical aspect of cleaning, the previous image on the drum must be cleared before a new one may be applied. The electrical cleaning of the OPC drum is performed by erasure lamps (usually corona wire technology) or a primary charge roller (PCR), which eliminate the previous latent image from the drum.
After the drum has been cleaned, it must be conditioned or charged to accept an image from a laser. A primary corotron (corona wire or PCR) applies a uniform negative charge (usually in the range of xe2x88x92600 V to xe2x88x92720 V DC) to the surface of the drum.
Following the conditioning stage is the writing stage. In this stage, a laser beam is used to discharge a conditioned charge to the drum surface. The conditioned charge creates a latent image on the drum. An aluminum base is connected to an electrical ground and the photoconductive material comprising the OPC drum becomes electrically conductive to ground when exposed to light (generally a laser). Therefore, the negative charges deposited onto the surface of the drum conduct to the aluminum base when exposed to light, creating the latent image. The latent image area will attract toner in a later stage.
The fourth stage is developing. At this stage, the latent image becomes a visible image. This stage generally requires four major components: toner particles, a developer roller assembly, a metering blade, and an AC/DC charge. The toner particles are attracted to the developer roller either by an internal magnet or by an electro-static charge. The developer roller carries the toner particles to a metering blade (a/k/a a doctor blade), where the toner particles tumble and create a tribo-electric charge (friction) on the surface of the toner particles. The metering blade then provides for an evenly distributed amount of toner particles to pass to the OPC drum. Once the toner particles have passed beyond the doctor blade, it is ready to be presented to the OPC drum. The developer roller is then charged with an AC/DC charge from a High Voltage Power Supply. This charge allows the toner particles to xe2x80x9cjumpxe2x80x9d from the developer roller and travel to the OPC drum where it is attracted to the latent image.
At this point, the toner image on the drum is transferred onto a sheet of paper. As the paper is passed under the OPC drum, it is passing over a transfer corotron assembly. The transfer corotron assembly places a positive charge on the back of the page, thus attracting the toner from the drum.
The sixth and final stage is fixing. Also known as fusing, this is the stage in which the toner is permanently affixed to the paper. A fuser assembly typically includes a heated roller, a pressure roller, a heating element, a thermistor, a thermal fuse, and, sometimes, a cleaning pad. The heating element is typically placed inside the heated roller, which is usually constructed of aluminum with a teflon coating. The heated roller is heated to approximately 355xc2x0 F. (180xc2x0 C.). The pressure roller (second roller) is usually a rigid foamed silicon rubber. This second roller applies pressure to the heated roller. The paper passes between the two rollers and the heated roller melts the toner particles while the pressure roller presses the toner into the fiber weave of the paper.
As laser printing technology has evolved, one of the primary focal points is printing speed. There is a constant demand for higher print speeds. However, as print speed increases, the power required for the fixing or fusing stage becomes greater, as the toner requires a certain amount of energy to melt and fuse to the paper. Current fusing technology has thus come to a speed xe2x80x9cceiling,xe2x80x9d where faster print speeds may require printers to have dedicated thirty-amp circuits to provide the necessary power to the heating element to keep up with the high print speeds. As speed demands continue to rise, the heating element power requirements to fuse the toner have become prohibitive. In addition, it has been a constant problem to apply an even heating distribution to the heated roller and the toner, leading to poorly fused images.
Further, during times when the printer is not in use, generally the user prefers that the printer, and especially the high energy absorbing heating element, revert to a low power or xe2x80x9csleepxe2x80x9d mode. However, when the user does have a need to print either while the printer is in sleep mode or when the printer has been turned off completely, it generally takes significant time for the heating element to warm up before the printer is operational. In addition, the use of heating elements introduces other deleterious effects, usually necessitating the use of cooling apparatus to keep components that may be heat sensitive from overheating. Often the use of fans is necessary--adding again to the power requirements and creating unwanted noise.
U.S. Pat. No. 5,212,526 (""526) discusses an alternative to the conventional fusing process by introducing an apparatus for simultaneously transferring and fusing a toner solute in a UV-curable solution. However, the apparatus taught by the ""526 patent utilizes a large belt to advance the page, and this large belt is very specialized and expensive. The large belt taught by the ""526 patent must be a photoreceptor and also must be capable of transmitting UV raysxe2x80x94capabilities that at present are very expensive to provide.
U.S. Pat. No. 5,232,812 (""812) discloses another alternative process for fusing an image. However, the process disclosed by the ""812 patent involves applying a separate layer of UV-curable liquid over the toner and does not cure the toner itself
In accordance with one aspect of the present invention, there is disclosed an apparatus including a print medium transport system and a first radio frequency energy emitting antenna for fusing toner to a print medium.
The apparatus may further include a print medium arranged adjacent the antenna, and toner disposed on the print medium. An oscillator may be coupled to the antenna and adapted to operate within the microwave range.
According to this aspect, the antenna may define a wave-guide, the wave-guide being shaped to focus the radio frequency energy emitted from the antenna to at least a portion of a line of an image or at least a portion of a line of text on the print medium.
In accordance with one aspect of the present invention, there is a second antenna, the first and second antennae being arranged on opposing sides of the print medium and providing a high frequency electromagnetic field thereacross. The first and second antennae may be arranged in a roller configuration, and the first and second antennae may apply contact pressure to the print medium as the print medium passes therebetween.
In accordance with one aspect of the invention, the print medium is paper.
In accordance with one aspect of the invention, the oscillator is adjusted to a frequency within 10% of a natural frequency of the toner.
In accordance with one aspect of the invention, there is described a method of fusing toner to a page, the method including the steps of passing the page by a radio frequency antenna and imparting electromagnetic energy to the toner on the page. According to this method, the radio frequency antenna may oscillate within 10% of a natural frequency of the toner, or the radio frequency antenna may oscillate within 1 MHz of the natural frequency of the toner. The electromagnetic energy imparted to the page is sufficient to raise the temperature of the toner to the toner melting point.
In one aspect of the present invention there is a printing apparatus including a photosensitive drum, a laser optic system for tracing an image on the photosensitive drum, a toner supply electrically charged opposite of the image traced on the photosensitive drum, and a radio frequency antenna for imparting electromagnetic energy to the toner.
In accordance with one aspect of the invention, there is disclosed a method of printing an image on a page by an electrophotographic process, the method including the steps of cleaning an organic photoconductive (OPC) drum, conditioning the OPC drum to accept an image from a laser, writing a latent image on the OPC drum with a laser beam, developing the latent image into a toner image by attracting toner to the OPC drum, transferring the toner to the page, and fusing the toner to the page by applying radio frequency energy to the toner. According to this method, there may be included the additional step of applying pressure to the toner and page substantially concurrent with the application of radio frequency energy to the toner.